Method and apparatus for use with an electron gun employing a thermionic source of electrons

ABSTRACT

A method includes providing an electron gun having a first head with a thermionic electron source and an accelerating electrode, and further includes replacing the first head with a second head having a power rating substantially different than that of the first head, and subsequently operating the electron gun without replacing the accelerating electrode. The electron gun may further include a platform spaced apart from the accelerating electrode and having an adjustably located locating member that engages a reference member on the head to position the head in three dimensions relative to the accelerating electrode. The platform may be adjustably spaced from the accelerating electrode in order vary the distance between the electron source and the accelerating electrode.

DESCRIPTION

[0001] 1. Related Application

[0002] The subject matter herein may be disclosed and/or claimed in U.S.patent application entitled “APPARATUS FOR AN ELECTRON GUN EMPLOYING ATHERMIONIC ELECTRON SOURCE”, attorney docket number EH-10218.

[0003] 2. Technical Field

[0004] This invention relates to an electron gun and more particularlyto an electron gun of a type having a thermionic source of electronsdisposed on a head, an accelerating electrode, and a platform to supportand position the head relative to the accelerating electrode.

[0005] 3. Background

[0006] Electron beam furnaces are used to heat materials to producevapors for deposition on an article. An electron beam furnace includesan electron gun, a deflection system, and a cooling system. The electrongun generates an electron beam. The deflection system directs theelectron beam toward the material to be heated. The cooling system coolsthe electron gun to prevent it from overheating.

[0007] The electron gun typically includes an electron source, afocusing electrode, and an accelerating electrode. The electron sourceis typically a cathode heated by an electric current to cause thecathode to emit electrons. The focusing electrode is typicallynegatively charged to repel the electrons and thereby direct theelectrons in a direction generally toward the accelerating electrode.The accelerating electrode is positioned downstream from the electronsource and the focusing electrode. The accelerating electrode istypically less negatively charged than the electron source and thefocusing electrode to cause the electrons to form into a beam and travelin the downstream direction.

[0008] In one known type of electron gun, the electron source and thefocusing electrode are elongated and disposed in a head. The head issupported by a platform spaced apart from the accelerating electrode.This type of electron gun is reliable and available in many differentpower ratings. The physical size of the head, the acceleratingelectrode, and the platform of a given one of these electron gunsdepends on its power rating.

[0009] It is important that an electron gun to be used in an electronbeam furnace generate an electron beam suitable for the type of materialto be heated and the type of deposition sought for the article.Different types of materials and depositions require electron beams ofdifferent amounts of electron beam power and may require differentelectron beam shape. However, it is desirable to have electron beam gunsoperate in a space charge limited mode. In such mode, the abovedescribed type of electron guns generally perform best at a power equalto or slightly below its power rating. Consequently, no one of theseelectron guns can adequately generate electron beams for all of theelectron beam powers required.

[0010] Whenever an electron gun in the electron beam furnace does notgenerate an electron beam suitable for the type of material to be heatedand the type of deposition sought for the article, the traditionalapproach has been to remove the electron gun and replace it with anelectron gun that provides a suitable electron beam. However completelyreplacing an electron gun can be difficult and time consuming. Moreover,the deflection system and the cooling system of the electron beamfurnace are connected to the accelerating electrode of the electron gunand are designed to suit the physical size of the accelerating electrodeand the power rating of the electron gun. Consequently, replacing theelectron gun requires replacing the deflection system and the coolingsystem, thereby compounding effort involved.

SUMMARY OF THE INVENTION

[0011] An object of the present invention is to reduce the effortinvolved with providing an electron gun of the above described type togenerate a suitable electron beam for a particular type of material anddeposition.

[0012] The present invention is predicated in part on the recognitionthat the accelerating electrode of a first electron gun having a firstpower rating can be operated with a head from a second electron gunhaving a second power rating substantially different than the firstpower rating, to provide an electron beam comparable to that which wouldbe provided by the second electron gun, and that such operation isfacilitated by making the platform adjustable enough to be able tosupport and position the head of the second electron gun, which may bephysically smaller than the head of the first electron gun.

[0013] According to a first aspect of the present invention, a methodincludes providing an electron gun having an accelerating electrode anda first head with a thermionic electron source, and further includesreplacing the first head with a second head having a power ratingsubstantially different than that of the first head, and subsequentlyoperating the electron gun without replacing the accelerating electrode.

[0014] This method reduces the effort associated with providing anelectron gun that generates an electron beam suitable for a particulartype of material and deposition. As used herein substantially differentmeans that one of the heads is at least twenty five percent less thanthe other of the heads. Using this method, suitable electron beams ofvarious power levels can be generated by replacing the head of anelectron gun without the need to replacing the accelerating electrode ofthe electron gun, thereby saving time. In one detailed embodiment, thepower rating of the second head is at least twenty five percent lessthan the power rating of the first head. In another detailed embodiment,the accelerating electrode is connected to a deflection system and acooling system, at least one of the deflection system and the coolingsystem is not replaced prior to operating the electron gun with thesecond head, thereby reducing the difficulty and the amount of timeconsumed.

[0015] According to a second aspect of the present invention, anapparatus for an electron gun has a head having a thermionic electronsource and at least one reference member, an accelerating electrode, anda platform spaced apart from the accelerating electrode and having atleast one locating member that engages the at least one reference memberof the head to position the head in three dimensions relative to theaccelerating electrode, wherein the at least one locating member isadjustably located and the location of the at least one locating membercan be adjusted by at least nine millimeters (mm).

[0016] Such apparatus is useful in practicing the above describedmethod, but is not limited to such. In order to operate the acceleratingelectrode of the first electron gun with the head of the second electrongun, it is desirable to be able to support and position the head on theplatform of the first electron gun. However, as described above, thehead of the second electron gun may be smaller in size than the head ofthe first electron gun. Providing the platform with locating membersthat are adjustably located by at least nine mm enables the platform tosupport and position heads of various sizes.

[0017] As described hereinbelow, although adjustably located locatingmembers are known, until now, their adjustability was limited to lessthan five mm, being merely intended to compensate for manufacturingtolerances of components of the electron gun and to facilitate alignmentof the electron source and the accelerating electrode.

[0018] According to a third aspect of the present invention, anapparatus for an electron gun has a head having a thermionic electronsource, an accelerating electrode, and a platform that supports the headand is adjustably spaced from the accelerating electrode.

[0019] This apparatus enables the shape of the electron beam to bevaried by varying the distance between the electron source and theaccelerating electrode. The apparatus is useful in practicing the abovedescribed method, but is not limited to such.

[0020] These and other objects, features and advantages of the presentinvention will become more apparent in the light of the followingdetailed description, accompanying drawings, and claims.

BRIEF DESCRIPTION OF THE DRAWINGS

[0021]FIG. 1 is a perspective, partially exploded, partially cut awayview of an electron gun;

[0022]FIG. 2 is perspective view of a thermionic electron source inaccordance with one embodiment of the present invention for use in theelectron gun of FIG. 1;

[0023]FIG. 3 is a cross section view in the direction of 3-3 of FIG. 2,of the thermionic electron source of FIG. 2;

[0024]FIG. 4 is a cross section view in the direction of 4-4 of FIG. 2,of the thermionic electron source of FIG. 2;

[0025]FIG. 5 is a cross section view in the direction of 5-5 of FIG. 2,of the thermionic electron source of FIG. 2;

[0026]FIG. 6 is a cross section view in the direction of 6-6 of FIG. 1,of the focusing electrode and the thermionic electron source used in theelectron gun of FIG. 1;

[0027]FIG. 7 is a side view of a prior art screw and a top view of aprior art spacer with an elongated hole;

[0028]FIG. 8 is a side view of a screw and a top view of a spacer of theelectron gun of FIG. 1;

[0029]FIG. 9 is a graph of a power density distribution of an electronbeam resulting from a prior art thermionic electron source; and

[0030]FIG. 10 is a graph of a power density distribution of an electronbeam resulting from the thermionic electron source of FIG. 2.

BEST MODE FOR CARRYING OUT THE INVENTION

[0031] The present invention is disclosed herein with respect to a bestmode embodiment for use in an electron gun illustrated in FIG. 1.Referring now to FIG. 1, an electron gun 20 for an electron beam furnace(not shown) has an accelerating electrode 22, a platform 24, and a head26. The electron gun 20 has a power rating of about sixty fivekilowatts. The electron gun 20 is representative of in shape, butlarger, than electron guns (not shown) having a power rating less thansixty five kilowatts, e.g., forty five kilowatts. The acceleratingelectrode 22 of the electron gun 20 has a plate portion 28 and a beamshaper portion 30. The beam shaper portion 30 is elongated and extendsin a longitudinal direction L. The accelerating electrode 22 further hasan elongated aperture 32 that extends in the longitudinal direction Land provides a path for electrons, generated by the head 26, to exit theelectron gun 20. The accelerating electrode 22 may comprise a coppermaterial and may be formed as one piece for example by milling.Alternatively, the accelerating electrode 22 may be an assembly whereinthe plate portion 28 comprises a stainless steel material and the beamshaper portion 30 comprises a copper material.

[0032] A plurality of bolts 34 connects the accelerating electrode 22 toa cooling plate 36 and a deflection system 38. Washer plates 40 recessedin the accelerating electrode 22 help distribute the load applied by thebolts 34. The cooling plate 36 is part of a cooling system, representedin part by a pair of water pipes 42. The cooling system 42 and thedeflection system 38 are specifically designed to suit the physical sizeof the accelerating electrode 22 and the power rating of the electrongun 20. For example, the cooling plate 36 is sized to contact as much ofthe surface area of the accelerating electrode 22 as is possible withoutcreating interference to other structures on the accelerating electrode22. This maximizes heat transfer between the accelerating electrode 22and the cooling system 42 and thereby helps prevent the electron gun 20from overheating. The cooling plate 36 and the deflection system 38 areillustrative of but physically larger than a cooling plate and adeflection system respectively suited to the physical size and the powerrating of an electron gun having a power rating of forty five kilowatts.Consequently, the cooling plate 36 and the deflection system 38 are toolarge to connect to the accelerating electrode of an electron gun havinga power rating of forty five kilowatts. The accelerating electrode 22may be electrically grounded by way of its connection to the coolingplate 36 and the deflection system 38.

[0033] The accelerating electrode 22 supports a first platform support44 and a second platform support 46. The first platform support 44comprises a high voltage insulator 50 and an insulator cover 52. Thehigh voltage insulator 50, which may comprise a ceramic material, has afirst end 54 and a second end 56. The first end 54 has a threaded stud58 that extends through a washer 60 and the accelerating electrode 22and engages a nut 62 to retain the high voltage insulator 50 to theaccelerating electrode 22. The second end 56 of the high voltageinsulator 50 has a shoulder 64 and a threaded stud 66. The shoulder 64abuts a collar 68 on the insulator cover 52. The threaded stud 66extends through the insulator cover 52 and engages a threaded cap 70 toretain the high voltage insulator 50 to the insulator cover 52. Theinsulator cover 52 limits formation of deposits on the high voltageinsulator 50. The insulator cover 52 further includes a threadedengagement surface 71. The second platform support 46 comprises a highvoltage insulator 72, an insulator cover 73, and an insulator cover 74,which are identical to the high voltage insulator 50, the insulatorcover 52, and the threaded cap 70 of the first platform support 44,respectively.

[0034] The first platform support 44 and the second platform support 46each further comprise a support ring, represented by a support ring 75.The support rings 75 each have a threaded engagement surface (not shown)and a support surface, represented by a support surface 76. The threadedengagement surfaces of the support rings 75 engage the threadedengagement surfaces of the insulator covers 52, 73 to retain the supportrings 75 to the insulator covers 52, 73. The support surfaces 76 of thesupport rings 75 provide support for the platform 24 and space theplatform 24 apart from the accelerating electrode 22. Retaining rings 78engage the insulator covers 52, 73 and retain the platform 24 on thesupport surfaces 76 of the support rings 75. Adjusting the spacingbetween one of the support surfaces 76 and the accelerating electrode 22is accomplished by loosening one of the support ring 75 and theretaining ring 78, and subsequently tightening the other of the supportring 75 and the retaining ring 78. Adjusting the spacing between one ormore of the support surfaces 76 and the accelerating electrode 22 ineffect adjusts the spacing between the platform 24 and the acceleratingelectrode 22.

[0035] The platform 24, which supports the head 26 of the electron gun40, has an opening 80. The platform 24 further has a first locatingmember 82 and a second locating member 84 disposed on opposite sides ofthe opening 80. The first locating member 82 comprises a spacer 86 and aprojection 88 extending therefrom. The second locating member 84comprises a spacer 90 and a projection 92 extending therefrom. Each ofthe projections may have a notch, represented by a notch 94, which arepart of a detent mechanism described hereinbelow. The spacer 86 of thefirst locating member 82 and the spacer 90 of the second locating member84 each have two holes, represented by a hole 96. Screws 98 extendthrough the holes 96 and engage the platform 24 to retain the firstlocating member 82 and the second locating member 84 to the platform 24.The screws 98 each have a head and a shank, represented by a shank 100.The shank 100 has a diameter 102. The holes 96 are preferably elongatedrelative to the diameter 102 of the shank 100 in a widthwise directionW, transverse to the longitudinal direction L, to provide a clearancebetween the spacers 86, 90 and the shank 100 of the screws 98. Thisclearance facilitates adjustment of the position of the locating memberrelative to the platform 24 and the accelerating electrode 22. Ifdesired, the holes 96 may be elongated in more than one directionrelative to the shank diameter 102. However, elongating the holes inonly one direction relative to the shank diameter helps to preventmisalignment between a thermionic electron source, describedhereinbelow, and the elongated aperture 32 of the accelerating electrode22. Repositioning of the first locating member 82 or the second locatingmember 84 is accomplished by loosening one or both of the screws 98 thatretain the locating member to the platform 24, positioning the locatingmember, and subsequently re-tightening the screws 98.

[0036] Referring now to FIG. 7, a prior art locating member 104A has aspacer 104B with a hole 104C that is elongated. The hole 104C has adimension 104D of less than ten mm in the widthwise direction W. Thehole 104C has a dimension 104F of about 6.5 mm in the longitudinaldirection L. A screw 104H employed in connection with the hole 104C toposition the locating member 104A has a head and a shank. The shank isthreaded and has a diameter 1041 of five mm. The dimensions of the screw104H and the hole 104C result in clearance between the spacer 104B andthe screw 104H and thereby result in adjustability of the locatingmember 104A. The adjustability in the widthwise direction W is less thanfive mm (less than 10 mm-5 mm). The adjustability in the longitudinaldirection L is about 1.5 mm (about 6.5 mm-5 mm). The adjustability inthe widthwise direction W and the adjustability in the lengthwisedirection L are intended to compensate for manufacturing tolerances ofcomponents of the prior art electron gun (not shown) and to facilitatealignment of the electron source and the accelerating electrode in theprior art electron gun. Consequently, as will be understood in view ofthe discussion hereinbelow, there is less than desired adjustabilitywith the prior art locating member 104A to position a head from anelectron gun that has a power rating of forty five kilowatts, which isphysically smaller than the head 26 of the electron gun 20.

[0037] Referring now to FIG. 8, in one embodiment of the presentinvention, the hole 96 has a dimension 106 at least fifteen mm in thewidthwise direction W. The hole 96 has a dimension 108 of six mm in thelengthwise direction L. The shank 100 of the screw 98 has a portion witha collar and a portion that is threaded. The portion with the collar hasa diameter 110 of 5.95 mm and a dimension 111 that is less than thethickness of the spacer 90. The portion that is threaded has a dimension109 of 5 mm to engage the platform 24 (FIG. 1). The dimensions of thescrew 98 and the hole 96 result in clearance between the spacer 90 andthe screw 98 and thereby adjustability of the locating member 84. Theadjustability in the widthwise direction W is at least nine nm (15 mm-6mm). The adjustability in the lengthwise direction L is 0.05 mm (6mm-5.95 mm). As will be evident in view of the discussion hereinbelow,the adjustability in this embodiment is enough to position the head 26of the electron gun 20, and enough to position a head from an electrongun having a power rating of forty five kilowatts.

[0038] Referring again to FIG. 1, the head 26 includes a frame member112 that is U shaped and comprises a stainless steel material. The framemember 112 has a first side wall 113 and a second side wall 114. Thefirst side wall 113 has a first reference member 116 having the shape ofa recess. The second side wall 114 has a second reference member 118having the shape of a recess. The first reference member 116 and thesecond reference member 118 have a distance D between them. The distanceD depends on the size of the head 26, which in turn depends on the powerrating of the electron gun 20. For the sixty five kilowatt electron gun20, the distance D is fifty mm, center to center. Note that for a fortyfive kilowatt electron gun, the distance between reference members isforty five mm, which is five mm (50 mm-45 mm) less than that of theelectron gun 20. When the frame member 112 is placed on the platform 24,the first reference member 116 and the second reference member 118engage the first locating member 82 of the platform 24 and the secondlocating member 84 of the platform 24, respectively, to position thehead 26 on the platform 24 and thereby positioning the head 26 in threedimensions relative to the accelerating electrode 22.

[0039] The frame member 112 may further have a pair of catch assemblies,represented by a catch assembly 122. Each catch assembly 122 has a ball124 and a spring 126. The catch assemblies 122 cooperate with thenotches 94 in the projections 88, 92 to define a detent mechanism thatretains the head 26 to the platform 24. For example, the ball 124engages the notch 94 in the projection 92 of the second locating member84. The spring 126 biases the ball 124 toward the notch 94. A pair ofscrews, represented by a screw 128, adjusts the bias provided by thesprings 126.

[0040] The head 26 further includes a first terminal 130, a secondterminal 132, and a thermionic cathode assembly 134. The first terminal130 engages the frame member 112. The second terminal 132 engages aconductor 136. The conductor 136 mechanically and electrically connectsthe second terminal 132 to a pair of non-magnetic, spring conductors 138that extend through the opening 80 of the platform 24 and support thethermionic cathode assembly 134. A plurality of screws 140 connects theconductor 136 to an insulator 142. Wedge shaped members 144 clamp theinsulator 142 to the frame member 112. A plurality of screws 146 biasesthe wedge shaped members 144 toward the insulator 142.

[0041] The thermionic cathode assembly 134 includes a thermionicelectron source 150 and a focusing electrode 152. The thermionicelectron source 150 and the focusing electrode 152 are spaced apart fromone another and each extends in the longitudinal direction L. Thethermionic electron source 150 is one piece and may comprise a tungstenmaterial. The focusing electrode 152 extends partially around thethermionic electron source 150 along a portion of a length 154 of thethermionic electron source 150. The focusing electrode 152 has a notch156 that extends in the longitudinal direction L. The notch 156 isbordered by a recessed surface 158. An ion trap 160 extendslongitudinally and into the notch 156 so as to be between the recessedsurface 158 of the focusing electrode 152 and the thermionic electronsource 150. The ion trap 160 is sacrificial in that it is expected thatthe ion trap 160 will be bombarded by ions and erode over time. The iontrap 160 reduces the amount of bombardment and erosion experienced bythe focusing electrode 152. The ion trap 160 is less costly to replacethan the focusing electrode and may comprise a carbon material. Notethat the opening 80 of the platform 24 is large enough for thethermionic cathode assembly 134 to pass through so as to facilitatepositioning the thermionic cathode assembly 134 proximate to theaccelerating electrode 22.

[0042] The head 26 further includes a first holder 162 and a secondholder 164. The first holder 162 is mechanically and electricallyconnected to the focusing electrode 152 by fasteners 166. The firstholder 162 has a clamping plate 168 and a screw 170. The screw 170engages the clamping plate 168 and thereby causes it to tightly engagethe thermionic electron source 150. The second holder 164 ismechanically and electrically connected to the frame member 112 byfasteners 172. The second holder 164 has a clamping plate 174 and ascrew 176. The screw 176 engages the clamping plate 174 and therebycauses it to tightly engage the thermionic electron source 150.

[0043] Referring now to FIGS. 2-5, the thermionic electron source has afirst end portion 180, a second end portion 182, and an aperturedisposed therebetween 184. The aperture 184 extends a portion of thelength 154 and a portion of a width 186 of the thermionic electronsource 150. The thermionic electron source 150 further comprises a firstlongitudinal portion 190 and a second longitudinal portion 192 thatextend in the longitudinal direction L and are spaced apart from oneanother by the aperture 184. The first end portion 180 and the secondend portion 182 rigidly join the first longitudinal portion 190 and thesecond longitudinal portion 192 together. The thermionic electron sourcemay be generally uniform in thickness 194. As illustrated, the aperture184 diminishes in width (i.e., tapers) near the ends of the firstlongitudinal portion 190 and the second longitudinal portion 192,although the aperture is not limited to such. The tapering helps toreduce buildup of stress and aids fabrication of the thermionic electronsource.

[0044] Referring also now to FIG. 6, the first longitudinal portion 190has a surface 196 that opposes the accelerating electrode 22; the secondlongitudinal portion 192 has a surface 198 that opposes the acceleratingelectrode 22. The surface 196 and the surface 198 may be inclined andface toward each other. Due to the incline, the thermionic electronsource 150 may have a widthwise cross section having the shape of achevron, as illustrated in FIG. 4. The incline of the surfaces 196, 198may diminish near the ends of the longitudinal portions in an effort tominimize stress. Making the surfaces 196, 198 inclined rather thancoplanar with each other is a way to increase to the width of theaperture without decreasing the surface area of the surfaces 196, 198.Note that the power rating of the electron gun 20 is related to thesurface area of the surfaces that face toward the accelerating electrode22. Depending on the incline and the width of the aperture 184, thethermionic electron source 150 may have almost as much surface areafacing toward the accelerating electrode 22 as the thermionic electronsource 20 would have in the absence of the aperture 184. The thermionicelectron source 150 and the ion trap 160 are preferably aligned with theelongated aperture 32 of the accelerating electrode 22, to maximize thebenefit of the aperture and the ion trap described below.

[0045] The thermionic electron source 150 may be fabricated using anysuitable method including but not limited to pressing, rolling, andmachining (including but not limited to electrical discharge machiningand laser machining) and combinations thereof. The thermionic electronsource 150 may be fabricated from a thermionic electron source that doesnot have an aperture 184 and has been used in the electron gun 20 andundergone ion bombardment.

[0046] For the electron gun 20, which has a power rating sixty fivekilowatts, the thermionic electron source 150 has a length 154 of onehundred mm and a width 186 of about three mm. The focusing electrode 152(FIGS. 1, 6) has a length of sixty five mm and a width of about thirtytwo mm. There is clearance 200 of about 0.5 millimeter between thethermionic electron source 150 and the focusing electrode 152. Thesacrificial ion trap 160 (FIGS. 1, 6) has a length equal to that of thefocusing electrode 152 and has a width in a range of about 1.5 mm toabout two mm. The length of the aperture 184 is about sixty mm, which isabout ten percent less than the sixty five millimeter length of thefocusing electrode 152. The width of the aperture 184 is about 0.75 mm,which is about one quarter of the width 186 of the thermionic electronsource 150. Note that the length 154 of the thermionic electron source150 and the length of the focusing electrode 152 typically depend on thepower rating of the electron gun, but the width 186 of the thermionicelectron source and the width of the focusing electrode 152 typical donot depend on the power rating of the electron gun. For example, for anelectron gun having a power rating of forty five kilowatts, thethermionic electron source 150 has a length 154 of eighty mm and a width186 of about three mm. The focusing electrode 152 has a length of fortyfive mm and a width of about thirty two mm.

[0047] In operation, the first terminal 130 and the second terminal 132are connected to a power supply (not shown). The power supply provides asource of electric current for the thermionic electron source 150. Theelectric current flows through the first terminal 130, the second holder164, the thermionic electron source 150, the first holder 162, thefocusing electrode 152, the spring conductors 138, the conductor 136,and the second terminal 132. As the electric current flows through thethermionic electron source 150 it results in heating thereof, to arelatively high temperature, but typically below the melting temperatureof tungsten, causing the thermionic electron source 150 to emitelectrons. The voltage across the thermionic electron source 150 istypically less than ten volts. Because the heating for the thermionicelectron source results from an electric current, the electron gun isreferred to as directly heated. A second power (not shown) provides thesecond terminal 132 with a negative voltage potential (typically about−20 kilovolts), which is in turn provided to the first holder 162 andthe focusing electrode 152 through the conductor 136 and the springconductor 138. The negative voltage potential causes the focusingelectrode 152 to repel the electrons and thereby direct the electrons ina direction generally toward the accelerating electrode 22. Theaccelerating electrode 22 is typically at an electrical ground voltagepotential by way of the connection between the accelerating electrode22, the cooling system 42, and the deflection system 38. Theaccelerating electrode causes the electrons to form into a beam andtravel in the downstream direction. The electron beam exits the electrongun 20 through the elongated aperture 32 of the accelerating electrode22.

[0048] The electron beam from the electron gun typically extends in thelongitudinal direction L and the widthwise direction W, and has agenerally rectangular cross section in a plane containing thelongitudinal direction L and the widthwise direction W. The electronbeam has a power density that varies across its width (i.e., in thewidthwise direction W). Referring now to FIG. 9, a graph illustrates apower density distribution obtained from the electron gun with a priorart thermionic electron source. The power density distribution hascharacteristics similar to that of a Gaussian distribution. Referringnow to FIG. 10, a graph illustrates a power density distributionobtained from the electron gun 20 with the thermionic electron source150. The power density distribution has characteristics similar to thatof a Gaussian distribution, but with some variation due to the aperture184 of the thermionic electron source.

[0049] The shape and the power density distribution of the electron beamdepends on the distance between the thermionic electron source 150 andthe accelerating electrode 22, and also depends on the differencebetween the voltage potential of the thermionic electron source 150 andthe voltage potential of the accelerating electrode 22. However, becausethe platform 24 is adjustably spaced from the accelerating electrode 22,various electron beam shapes and various power density distributions maybe obtained by moving the platform 24 closer to or farther away from theaccelerating electrode 22, without the need to vary the voltagepotential between the thermionic electron source and the acceleratingelectrode. There is preferably at least one inch of adjustability tomake possible a wide range of electron beam shapes and power densitydistributions

[0050] The electron beam is used in the electron beam furnace (notshown) to vaporize materials for deposition on articles. Positivelycharged ions are produced as a result of the vaporization and of thematerial in the electron beam furnace. Some of these ions have adirection of travel opposite that of the electrons in the electron beamcausing the ions to travel through the elongated aperture in theaccelerating electrode and toward the thermionic electron source. Theions have the potential to bombard and erode the thermionic electronsource.

[0051] However, because the thermionic electron source has an aperture,many of these positively charged ions do not strike the thermionicelectron source 150, but rather pass through the aperture and strike thesacrificial ion trap. The thermionic electron source 150 is thus lesssusceptible to ion bombardment and thus has a longer operating life thanprevious ribbon type thermionic electron sources. The life expectancy ofthe thermionic electron source depends on the operating conditions,however, for a given set of operating conditions, the life expectancy ofthe thermionic electron source is about two times greater than it wouldbe without the aperture 184. Moreover, because the improved electronsource is one piece, use of the electron source does not require supportand relative positioning of multiple emitters such as that required byan electron source having two separate and parallel emitters. Inaddition, the one piece construction may make the electron source morerigid and thus more durable and less likely to deform than an electronsource having two separate emitters.

[0052] As stated above, it is important that an electron gun to be usedin an electron beam furnace generate an electron beam suitable for thetype of material to be heated and the type of deposition sought for thearticle. Different types of materials and depositions require electronbeams of different amounts of electron beam power and may requiredifferent electron beam shape. It is also desirable to have the electronbeam gun operate in a space charge limited mode. However, the sixty fivekilowatt electron gun operates in space charge limited mode within apower range from about forty three kilowatts to about seventy kilowatts.Consequently, the sixty five kilowatt electron gun cannot adequatelygenerate electron beams for all of the electron beam powers required.

[0053] It has been determined that the accelerating electrode 22 of theelectron gun 20 can be operated with the head 26 of the electron gun 20or alternatively with the head of a second electron gun having a powerrating at least twenty five percent less than the sixty five kilowattpower rating of the electron gun 20. In this alternative, the electronbeam that results is comparable to that which would be generated by thesecond electron gun. For example, an electron gun comprising theaccelerating electrode 22 of the sixty five kilowatt gun and the head ofa forty five kilowatt gun generates an electron beam comparable to thatgenerated by the forty five kilowatt electron gun. A forty five kilowattelectron gun operates in space charge limited mode within a power rangefrom about twenty seven kilowatts to about forty eight kilowatts. Theaccelerating electrode 22 of the electron gun 20 and the head of a fortyfive kilowatt electron gun operate together in space charge limited modewithin the same range as that of the forty five kilowatt electron gun.Since the accelerating electrode 22 is not replaced and the resultingelectron gun operates at a power less than the power rating of theelectron gun 20 (sixty five kilowatts), there is no need to replace thedeflection system 38 or the cooling system 42. Therefore, suitableelectron beams of various power levels can be generated withoutreplacing the accelerating electrode 22 of the electron gun 20, thedeflection system 38 or the cooling system 42, thereby reducing theeffort involved.

[0054] Providing locating members 82, 84 that are adjustably located byat least nine mm facilitates the operation of the accelerating electrode22 of the electron gun 20 with the head of a second electron gun thatmay be smaller in size than the head 26 of the sixty-five kilowattelectron gun 20. The distance between the first locating member and thesecond locating member 84 can be sufficiently varied so as to correspondto the distance D between the reference members of the head of thesecond electron gun. For example, as stated above, for a forty-fivekilowatt electron gun, the distance between reference members on thehead is forty-five mm, which is five mm (50 mm-45 mm) smaller than thatof the electron gun 20. In contrast, the prior art locating member 104Ahas adjustability of less than five mm and is intended to compensate formanufacturing tolerances of components of the prior art electron gun andto facilitate alignment of the electron source and the acceleratingelectrode in the prior art electron gun. Consequently, there is lessthan desired adjustability with the prior art locating member 104A toposition a head that is five mm smaller than the head 26 of the electrongun 20.

[0055] In regard to the locating members 82, 84 and the referencemembers 116, 118, although disclosed with respect to two locatingmembers and two reference members, all that is required is at least onelocating member and at least one reference member. The adjustability maybe whatever is appropriate to facilitate support and positioning ofheads from other electron guns. The spacing is preferably at least ninemm so as to accommodate the different distances encountered between thereference members of the head 26 of the electron gun 20 and thereference members of the head of an electron gun having a power ratingof forty-five kilowatts. The locating members may comprise recesses orprojections.

[0056] In regard to the platform supports 44, 46, although shown withtwo platform supports, all that is required is at least one platformsupport. The platform supports need not comprise an insulator 50 and aninsulator cover 52. Any suitable type of engagement surface may be used.The platform supports need not be the same as each other. Althoughdisclosed as having a support ring 75 that engages an insulator cover52, the platform supports 44, 46 are not limited to such and may employa support member of any shape, including, but not limited to, one ormore pins.

[0057] However, it should be understood that the adjustable locatingmembers 82, 84 and the adjustable platform supports 44, 46 are notrequired to practice the method of the present invention. Thus, theaccelerating electrode 22 of the electron gun 20 can be operated withthe head of a second electron gun without the presence of the adjustablelocating members 44, 46 or the adjustable platform supports 82, 84. Forexample, the head of the second electron gun may be modified to fit onthe platform, the platform may be modified in some other way to supportand position the various heads, or a plurality of different platformsmay be employed.

[0058] Although shown with one focusing electrode 152 and oneaccelerating electrode 22, there may be any number of focusing andaccelerating electrodes. The thermionic electron source 150 need nothave an aperture 184. Nor must there be a sacrificial ion trap 160.

[0059] Furthermore, although described with respect to an electron gun20 having a power rating of sixty-five kilowatts, the present inventionmay be used with electron guns of any power rating.

[0060] While the present invention has been described with reference toa best mode embodiment, this description is not meant to be construed ina limiting sense. Various modifications of the best mode of embodiment,as well as additional embodiments of the invention, will be apparent topersons skilled in the art upon reference to this description, withoutdeparting from the spirit of the invention, as recited in the claimsappended hereto. It is therefore contemplated that the appended claimswill cover any such modifications or embodiments as fall within the truescope of the invention.

What is claimed is:
 1. A method comprising: providing an electron gunhaving a first head and an accelerating electrode, the first head havinga power rating and a thermionic electron source; replacing the firsthead with a second head, the second head having a power rating and athermionic electron source, one of the power rating of the first headand the power rating of the second head being at least twenty-fivepercent less than the other of the power rating of the first head andthe power rating of the second head; and subsequently operating theelectron gun without replacing the accelerating electrode.
 2. The methodof claim 1 wherein the power rating of the second head is at leasttwenty-five percent less than the power rating of the first head.
 3. Themethod of claim 2 wherein the thermionic electron source of the firsthead and the thermionic source of the second head each extends in alongitudinal direction.
 4. The method of claim 2 wherein the powerrating of the second head is at least thirty percent less than the powerrating of the first head.
 5. The method of claim 2 wherein theaccelerating electrode is connected to a cooling system and a deflectionsystem and the step of subsequently operating is carried out withoutreplacing greater than one of the cooling system and the deflectionsystem.
 6. The method of claim 5 wherein the step of subsequentlyoperating is carried out without replacing either of the cooling systemand the deflection system.
 7. The method of claim 6 wherein thethermionic electron source of the first head, the thermionic source ofthe second head, and the accelerating electrode each extends in alongitudinal direction.
 8. The method of claim 1 wherein the first headhas at least one reference member and the second head has at least onereference member, the electron gun further has a platform that supportsthe head and has at least one adjustably located locating member thatengages the at least one reference member of the first head, thelocation of the at least one locating member is adjustable by at leastnine millimeters, and the step of replacing further includes adjustingthe location of the adjustably located locating members so that theplatform supports the second head and the at least one adjustablylocated locating member engages the at least one reference member of thesecond head.
 9. The method of claim 1 wherein the electron gun furthercomprises at least one platform support supported by the acceleratingelectrode and having a support surface that is adjustably spaced fromthe accelerating electrode and supports the platform, and the step ofreplacing further includes adjusting the spacing between theaccelerating electrode and the platform.
 10. Apparatus for an electrongun, the apparatus comprising: a head having a thermionic electronsource that extends in a longitudinal direction and further having atleast one reference member; an accelerating electrode; and a platformpositioned spaced apart from the accelerating electrode and having atleast one locating member adjustably located on the platform, thelocation of the at least one locating member being adjustable by atleast nine millimeters in a direction transverse to the longitudinaldirection, wherein the at least one reference member engages the atleast one locating member to position the head in three dimensionsrelative to the accelerating electrode.
 11. The apparatus of claim 10wherein the one of the at least one reference member and the at leastone locating member comprising a projection, the other of the at leastone reference member and the at least one locating member comprising arecess.
 12. The apparatus of claim 10 wherein the at least one referencemember of the head comprises two recesses and the at least one locatingmember of the platform comprises two projections.
 13. The apparatus ofclaim 10 wherein the at least one locating member of the platformcomprises a spacer having two holes, the platform further having screws,each of the screws having a head and a shank, the shank having adiameter, the shank of the screws extending through the holes and engagethe platform, the holes being elongated relative to the diameter of theshank to provide a clearance between the spacer and the shank tofacilitate adjustment of the position of the locating member relative tothe platform and the accelerating electrode.
 14. The apparatus of claim10 wherein the at least one reference member of the head comprises tworecesses and the at least one locating member of the platform comprisestwo locating members, each of the locating members comprising a spacerand a projection, each of the spacers having two holes, the platformfurther having screws, each of the screws having a head and a shank, theshank having a diameter, the shank of the screws extending through theholes and engage the platform, the holes being elongated relative to thediameter of the shank to provide a clearance between the spacer and theshank to facilitate adjustment of the position of the locating memberrelative to the platform and the accelerating electrode.
 15. Apparatusfor an electron gun, the apparatus comprising: a head having athermionic electron source that extends in a longitudinal direction; anaccelerating electrode; a platform that supports the head; and at leastone platform support supported by the accelerating electrode and havinga support surface that is adjustably spaced from the acceleratingelectrode and supports the platform.
 16. The apparatus of claim 15wherein the at least one platform support comprises an insulator, aninsulator cover, and a support member, the insulator cover is retainedto insulator and has an engagement surface, and the support member hasthe support surface disposed thereon and engages the engagement surfaceto adjustably space the support surface from the accelerating electrode.17. The apparatus of claim 16 wherein the engagement surface of theinsulator cover comprises a plurality of threads, and the support memberis a support ring with a plurality of threads that engage the threadedengagement surface of the insulator cover.
 18. The apparatus of claim 15wherein the head has at least one reference member and the platform hasat least one locating member adjustably located on the platform, the atleast one locating member having a location on the platform, thelocation of the at least one locating member being adjustable by atleast nine millimeters, wherein the at least one reference memberengages the at least one locating member to position the head in threedimensions relative to the accelerating electrode.
 19. The apparatus ofclaim 17 wherein the head has at least one reference member and theplatform has at least one locating member adjustably located on theplatform, the at least one locating member having a location on theplatform, the location of the at least one locating member beingadjustable by at least nine millimeters, wherein the at least onereference member engages the at least one locating member to positionthe head in three dimensions relative to the accelerating electrode. 20.The apparatus of claim 15 wherein the at least one reference member ofthe head comprises two recesses and the at least one locating member ofthe platform comprises two locating members, each of the locatingmembers comprising a spacer and a projection, each of the spacers havingtwo holes, the platform further having screws, each of the screws havinga head and a shank, the shank having a diameter, the shank of the screwsextending through the holes and engage the platform, the holes beingelongated relative to the diameter of the shank to provide a clearancebetween the spacer and the shank to facilitate adjustment of theposition of the locating member relative to the platform and theaccelerating electrode.
 21. The apparatus of claim 17 wherein the atleast one reference member of the head comprises two recesses and the atleast one locating member of the platform comprises two locatingmembers, each of the locating members comprising a spacer and aprojection, each of the spacers having two holes, the platform furtherhaving screws, each of the screws having a head and a shank, the shankhaving a diameter, the shank of the screws extending through the holesand engage the platform, the holes being elongated relative to thediameter of the shank to provide a clearance between the spacer and theshank to facilitate adjustment of the position of the locating memberrelative to the platform and the accelerating electrode.